In the art of motor vehicle lighting, it is well known to provide headlamps with means for limiting the light beam to have a so called cut-off so that its rays cannot blind the driver of a vehicle moving in the opposite direction.
There are precise standards regulating the various amounts of lighting which are permissible at different points in space around the optical axis of the headlamp. In testing for this purpose, the illumination provided by the headlamp is located on a screen, which is usually at right angles to the optical axis and at a given distance from the headlamp. Each point of the image on the screen can be located by the vertical and horizontal angular distance (relative to the projector) between it and the central point on the optical axis of the headlamp.
Usually, the headlamp beam is cut off in such a manner that the screen shows a very marked separation between a bright illuminated region and a dark non-illuminated region, the separation forming a material representation of the cut-off. According to European Standards laid down in 1957, (for left-hand drive vehicles) the cut-off appears as a bent line comprising a horizontal segment at the left of the central axis of the headlamp, and a segment sloping upwards at 15.degree. to the right of the central axis. Other forms of cut-off may be used, e.g. rectilinear cut-off (before 1957) or "offset" cut-off, as described by the Applicants in their French Pat. No. 7346304.
Basically, the problem of determining the position of the cut-off on a vehicle headlamp is part of the problem of adjusting the headlamps, either when first installed on the vehicle or during use.
Numerous checking and/or adjusting devices have been proposed and used. They nearly all comprise a screen for intercepting the light beam to be checked, and means for moving the screen relative to the headlamp, see e.g. French Pat. Nos. 825653 and 1087394.
In the simplest devices, the screen comprises a material representation of the cut-off, which is determined by making the light-shade limit appearing on the screen coincide with the material representation. Either the screen can be moved relative to a reference position to locate the cut-off or, as is done inter alia in devices sold in France under the trademark "REGLOSCOPE", the headlamp can be adjusted so that the projected cut-off coincides with the represented cut-off.
In some more elaborate devices, it has been proposed to use photoelectric detectors to convert the light intensities at different points on the screen into useful electric signals. If a screen on to which a cut-off beam is projected is vertically scanned upwards, the light intensity varies very rapidly from vey high to very low when the cut-off is crossed. Thus, if the variation in light intensity with height is measured by means of one or more detectors, it is theoretically easy to locate the cut-off along any vertical segment of the screen.
In practice, for obvious economic requirements, the number of detectors and the electronic apparatus for processing the resulting electric signals must be reduced to a minimum, and the cut-off must be determined in a manner corresponding as closely as possible to its real position.
All the devices known hitherto are relatively complex and expensive, since they usually search for the point at which the second or third derivative of the light intensity with respect to height becomes equal to zero. It may be assumed, for example, that the second derivative is zero at the cut-off, corresponding mathematically to a point of inflexion on the intensity/height curve.
The invention provides a method and device for determining the cut-off by means of photoelectric detectors, in a manner which is much simpler and much more efficient than the prior-art devices.